1. Field of the Invention
The present invention relates to a planetary transmission having a continuously variable transmission ratio.
2. Description of the Related Art
Transmissions having a continuously variable transmission ratio, which are also referred to as variable speed drives, are known in the form of belt-driven transmissions on the one hand, for example belt-driven conical-pulley transmissions in which the power is transmitted via traction means such as V-belts, plate-link chains, or thrust link chains, and on the other hand as frictional transmissions. Examples of frictional transmissions are toroidal transmissions and conical ring transmissions. The planet wheels in such frictional transmissions must be supported, tilted, and clamped by means of components provided especially for those purposes.
A planetary friction wheel transmission is also known in which a planet carrier is not absolutely necessary, since the planet wheel circumferential surfaces and the sun wheel circumferential surface in combination with the inner circumferential surface of the ring wheel and the mutual frictional contact are so shaped that the planet wheels are held axially by the frictional contact with the sun wheel and the ring wheel.
At the same time, a system is provided in a known planetary transmission having a continuously variable transmission ratio, in which two sun wheels are provided that are rotatable around the same axis of rotation at an axial distance from each other, both of which have sun wheel circumferential surfaces. One of the sun wheels can be connected to a shaft in a rotationally fixed connection, while the other sun wheel is rotatably supported on the shaft and biased in the direction of the firmly connected sun wheel, so that the necessary clamping between the sun wheels and the planet wheels for a frictional transmission is achieved.
Furthermore, in the planetary friction transmission a ring wheel situated on the same axis with the sun wheels is provided having a radially inner internal circumferential surface, as well as planet wheels having planet wheel circumferential surfaces that are each in frictional contact with the ring wheel internal circumferential surface and the sun wheel circumferential surfaces. The ring wheel can be shifted axially relative to the sun wheels, in which case the sun wheel circumferential surfaces, the planet wheel circumferential surfaces and the internal circumferential surface of the ring wheel are formed in such a way that when the ring wheel is shifted axially relative to the sun wheels, the axes of rotation of the planet wheels are tipped relative to the axis of rotation of the sun wheels, and the transmission ratios between the ring wheel and each of the sun wheels change in the opposite direction. To that end, the planet wheel circumferential surfaces, which are in frictional contact with the sun wheels, taper toward the end faces of the planet wheels, and the sun wheel circumferential surfaces taper toward the end faces of the sun wheels that are facing each other.
Alternatively, the planet wheel circumferential surfaces taper in the direction away from the end faces of the planet wheels, and the sun wheel circumferential surfaces taper from the end faces of the sun wheels facing away from each other. Furthermore, with that design a separator element is introduced, so that the planet wheels are held at substantially the same circumferential distance from one another. The separator element does not influence the tipping possibility of the planet wheels. It extends into intermediate spaces between the planet wheels and into a groove provided on the planet wheels between the planet wheel circumferential surfaces, which are in frictional contact with the sun wheel circumferential surfaces.
During the transmission of torque between input and output drives in such a planetary friction transmission, a tipping moment or tilt moment is produced because of the frictional contact of the planet wheels with the sun wheel and the ring wheel, which moment twists the planet wheels around an axis perpendicular to their axes of rotation. The contours of the planet wheel circumferential surfaces and of the internal circumferential surface of the ring wheel are so shaped that the tipping moment shifts the planet wheels radially inward and reinforces the clamping between the sun wheel circumferential surfaces, the planet wheel circumferential surfaces, and the internal circumferential surface of the ring wheel. The tipping of the rolling elements or planet wheels under the influence of the drive torque can be utilized for self-acting, torque-dependent clamping. Of course, the tipping of the planet wheels produces skewing at the contact points, which can result in slippage and cause losses (a temperature increase at the contact points), so that overall a reduction of the transmittable useful torque results.
Starting from that background, an object of the present invention is to provide a planetary transmission having a continuously variable transmission ratio, and which improves the efficiency of a rolling planetary transmission of the type described earlier and which improves the flexibility of utilization of the planetary transmission, in particular in regard to multiple adjustable transmission ratio ranges.